1. FIELD OF THE INVENTION
The present invention relates to improvements for water cooling plasma arc working apparatuses.
2. DESCRIPTION OF THE PRIOR ART
In a conventional water cooling plasma arc working apparatus, water for cooling a working torch thereof is supplied, via a stop cock, from a source of industrial water or city water or by a so called cooling water circulator for supplying cooling water from a reservoir tank to respective working torches using a water supplying pump.
FIG. 10 shows a conventional water cooling plasma arc working apparatus with a cooling water circulating system.
As shown therein, a cooling water circulator 1 provided with a reservoir tank 11, a water supplying pump 12 and an electric motor 13 for driving the pump 12 water cooling plasma arc working torch 5 is cooled with cooling water supplied by the water pump 12 through a supplying conduit 14 such as a hose. Hot water having used for cooling the torch is discharged through a discharging conduit 15, to the reservoir tank 11. A working power source unit 2 for supplying working power provides an electric power transformer circuit for transforming commercial power to a direct current or suitable for plasma are working, a control circuit for controlling start and stop of the power supply and the supply of plasma-forming gas and an electro-magnetic valve 201 for starting and stopping the supply of plasma-forming gas. The working power source unit 2 and the plasma arc torch 5 are connected with a torch cable 4 including a power cable, a supply hose for plasma-forming gas, a signal cable for a torch switch 501 for manually operating start and stop of the plasma arc working and circulating hoses 14 and 15 for cooling water. A work piece 6 is connected to one of output terminals of the power source unit 2 by another power cable. This power source unit 2 is connected, via a connecting cable 8, to a commercial alternating current source of single or three phases. The water pump 12 of the cooling water circulator 1 is started before the start of working operation and is kept running until the end of daily operation.
FIG. 11 shows a connection circuit of the conventional apparatus shown in FIG. 10 with external devices.
A reference numeral 7 denotes a power switch and, when it is switched on, the alternating electric power is supplied to the working power source unit 2 and the electric motor 13 for driving the water pump 12 and, accordingly, the supply of cooling water is started.
Another reference numeral 202 indicates the control circuit of the working power source unit 2 to which the electro-magnetic valve 201 for supplying plasma-forming gas and the torch switch 501 are connected.
As is well known to those skilled in the art, the torch 5 provided with a main electrode 52 and a tip electrode 55 having a passage for cooling water therein and, thereby, it is cooled.
In operation of the apparatus shown in FIGS. 10 and 11, when the torch switch 501 is switched on, the control circuit 202 activates the electro-magnetic valve 201 to start the supply of plasma-forming gas. After a predetermined time interval, the working electric power is supplied between the torch 5 and the work piece 6 and desired working operation is started after the well-known plasma arc starting process.
If the torch switch 501 is switched off upon finishing the working operation, the electric power supply is suspended to cut plasma arc and, after a predetermined time interval, the electro-magnetic valve 201 is closed to stop the supply of plasma-forming gas.
Further, the supply and stop of cooling water is controlled by operating the stop cock 16 manually.
FIG. 12 shows a cross-section of a tip portion of a conventional plasma arc torch 5 of water cooling type which provided with a protection cap 57 for covering the tip electrode 55 from outside thereof and a detection means for detecting a mounted state of the protection cap 57.
In FIG. 12, reference numerals 51 to 55 respectively denote electrodes and electrode support members made of an electrically conductive material, a main electrode 52 supported on the tip portion of the first electrode support member 51; an insulation sleeve arranged therearound, a second electrode support member arranged around the insulation sleeve 53 and a tip electrode 55 supported on the tip portion of the second electrode support member 54 which provides a jet hole 551 for spouting a plasma jet at the center portion of the tip thereof. Further, reference numerals 56, 57 and 58 denote a torch body made of an insulating material, the protection cap covering the tip electrode 55 from outside and a conduit for cooling water. The cooling water flowing from the supplying hose 14 cools the main electrode 52 directly and thereafter, is drained from the torch 5 through the drain hose 15 after flowing passages indicated by arrows in FIG. 12. Gas for forming a plasma arc such as pressurized air or oxygen is supplied into a space defined between the main electrode 52 and the tip electrode 55, as indicated by an arrow in FIG. 12 and then, spouted from the jet hole 551.
Further, there are provided with a pair of detection mechanisms 66 and 66 at the tip portion of the torch 5. Each is comprised of a terminal element 62 to which a lead line 61 is connected, a detection pin 63 movable in the axial direction (vertical direction) of the torch 5, and a compression spring 64 arranged between the terminal element 62 and the detection pin 63 and an O-ring 65 for restricting an excessive downward displacement of the pin 63 in Y.sub.1 direction.
In the above mentioned structure, when the protection cap 57 is mounted on the tip of the torch body 56, it pushes each detection pin 63 upwardly (in the Y2 direction) against the spring force of the spring 64. Each detection pin 63 contacts with the corresponding terminal element 62 via a spring 64. As the result, the pair of detecting mechanisms 66 and 66 are electrically connected with each other by a conductive layer having been applied on the upper end of the protection cap 57. Only in this conductive state between the pair of detection mechanisms 66 and 66, the working operation is allowed to start.
On the other hand, when the protection cap 57 is dismounted from the torch 56, each detection pin 63 is moved downwardly (in the Y1 direction) by each spring 64 until stopped by the O-ring 65 as a stopper and, thereby, the pins 63 and 63 are brought into an electrically disconnected state with each other. Thus, each detection and, thereby, a detection signal is outputted to the control circuit to cut off the power supply to the electrodes 52 and 55.
In the operation of the torch shown in FIG. 12, a high voltage of a high frequency generated by a high frequency generator 67 is applied, via a capacitor 68, between the main electrode 52 and the tip electrode 55 to generate a so-called pilot arc. This pilot arc is spouted from the jet hole 551 of the tip electrode 55 by the action of a flow of the plasma forming gas. When the torch 5 is brought near the work piece 6 while keeping the pilot arc, a working arc is generated between the main electrode 52 an the work 6. Once the working arc has been generated, the pilot arc disappears because of a resistance 69 connected on the way of the current path for generating the pilot arc. The high frequency generator 67 is stopped when the pilot arc is generated once.
In the conventional plasma arc working apparatus as shown in FIGS. 10 to 12, the water pump 12 for supplying cooling water is driven always regardless to the actual working operation and, due to the high duty ratio thereof, a high capacity is needed and the life thereof is extremely shortened.
Further, if it becomes necessary to clean the hoses 14 and 15 or to exchange the main electrode 52 and/or the tip electrode, the stop cock 16 is operated to stop the supply of cooling water at first. However, such a maintenance operation as mentioned above is troublesome because an operator has to drain remaining water from the hoses 14 and 15 and the torch 5 in order to avoid an accidental leak of water. Of course, the power supply to the torch 5 is cut off by operating the manual torch switch in order to avoid an accidental electric shock upon the maintenance operation. In the type of torch shown in FIG. 12, the power supply to the torch 5 is automatically cut off by a detection signal which is generated by the pair of detection mechanisms 66 and 66 when the protection cap 57 is removed from the torch body 56.
Further, in the conventional water cooling plasma arc working apparatus, the circulated water is discharged from the reservoir tank 11 downwardly. Due to this, as shown schematically in FIG. 13, water in the tank 11 is apt to scatter outside from an exit of air thereof in operation of the cooling water circulator. Such a leak of water is dangerous because it may invite slip accidents, corrosion of other equipments and/or electrical shocks.